Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet or continuous ink jet.
The first technology, drop-on-demand ink jet printing, typically provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. With thermal actuators, a heater, located at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink droplet to be expelled. The bubble then collapses as the heating element cools, and capillary action draws fluid from a reservoir to replace ink that was ejected from the nozzle.
Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel, on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes in an applied electric field forcing an ink droplet out of a nozzle. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead meta-niobate.
Many other types of drop on demand actuators have been disclosed. In U.S. Pat. No. 4,914,522, which issued to Duffield et al. on Apr. 3, 1990, a drop-on-demand ink jet printer utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an ink nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied for controllable time periods at a constant pressure through a conduit to a control valve. The ink dot size on the image remains constant while the desired color density of the ink dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print medium.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This early technique is known as electrostatic binary deflection continuous ink jet.
U.S. Pat. No. 4,636,808, issued to Herron et al., U.S. Pat. No. 4,620,196 issued to Hertz et al. and U.S. Pat. No. 4,613,871 disclose techniques for improving image quality in electrostatic continuous ink jet printing including printing with a variable number of drops within pixel areas on a recording medium produced by extending the length of the voltage pulses which charge drops so that many consecutive drops are charged and using non-printing or guard drops interspersed in the stream of printing drops. Additionally, U.S. Pat. No. 6,003,979, issued to Schneider et al. on Dec. 21, 1999, discloses grouping of guard drops and printing drops in droplet streams so that some groups have no guard drops interspersed between a particular number of printed drops.
Later developments for continuous flow ink jet improved both the method of drop formation and methods for drop deflection. For example, U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitude stimulations resulting in longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher.
U.S. Pat. No. 6,588,888 entitled “Continuous Ink-Jet Printing Method and Apparatus” issued to Jeanmaire et al. discloses a continuous ink jet printer capable of forming droplets of different size and with a droplet deflector system for providing a variable droplet deflection for printing and non-printing droplets.
Typically, continuous ink jet printing devices are faster than drop-on-demand devices and are preferred where higher quality printed images and graphics are needed. However, continuous ink jet printing devices can be more complex than drop-on-demand printers, since each color printed requires an individual droplet formation, deflection, and capturing system.
Briefly referring to FIG. 1a, a continuous ink jet printer system 10 includes an image source 50 such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. Image data image processor 60 is stored in image memory 80 and is sent to droplet controller 90 which generates patterns of time-varying electrical pulses to cause droplets to be ejected from an array of nozzles on print head 16, as will be described. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that drops formed from a continuous ink jet stream will form spots on a recording medium 18 in the appropriate position designated by the data in image memory 80.
Referring to FIG. 1b, a representative prior art continuous inkjet printhead 16 (U.S. Patent Application Publication No. US 2003/0202054) is shown schematically. Ink 19 is contained in an ink reservoir 28 under pressure. The ink is distributed to the back surface of print head 16 by an ink channel 30 in silicon substrate 15. The ink preferably flows through slots and/or holes etched through silicon substrate 15 of print head 16 to its front surface, where a plurality of nozzles 21 and heaters 22 are situated. In the non-printing state, continuous ink jet non-printing droplets 40 deflected by drop deflection means 48 and are unable to reach recording medium 18 due to an ink gutter 17 that blocks the non-printing droplets. Printing droplets 38, which are shown larger than non-printing droplets in FIG. 1b, are deflected only slightly by drop deflection means 48 and therefore miss gutter 17 and reach recording medium 18. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to ink reservoir 28 under the control of ink pressure regulator 26, FIG. 1a. 
One well known problem with any type of inkjet printer, whether drop-on-demand or continuous flow, relates to precision of dot positioning. As is well known in the art of inkjet printing, one or more droplets are generally desired to be placed within pixel areas (pixels) on a receiver, the pixel areas corresponding, for example, to pixels of information comprising digital images. Generally, these pixel areas comprise either a real or a hypothetical array of squares or rectangles on the receiver, and printed droplets are intended to be placed in desired locations within each pixel, for example in the center of each pixel area, for simple printing schemes, or, alternatively, in multiple precise locations within each pixel area to achieve half-toning. If the placement of the droplets is incorrect and/or their placement cannot be controlled to achieve the placements desired within each pixel area, image artifacts may occur, particularly if similar types of deviations from desired locations repeat in adjacent pixel areas.
Incorrect placement of droplets may occur due to manufacturing variations between nozzles or to dirt or debris in or near some nozzles. Slight nozzle differences affect the trajectory direction of droplets ejected from a printhead, either in the direction in which the print head is scanned (fast scan direction) or in the direction in which the receiving medium is periodically stepped (slow scan direction, usually orthogonal to the fast scan direction). Slight errors in trajectory result in corresponding placement errors for printed drops. Another possible error source for dot placement is response time, which can be slightly different between nozzles in an array, resulting in displacement errors in the fast scan direction. That is, each nozzle in an array may not emit its dot of printing ink with precisely the same timing. As a result of such fabrication differences and timing response, dot positioning on the print medium may vary slightly, pixel to pixel, with respect to the desired positioning. For the most part, these minor differences result in error distances that are some fraction of a pixel dimension. For example, where pixels may be placed 30 microns apart, center-to-center, typical errors in dot placement are on the order of 2 microns or larger.
Under some conditions, small placement errors within this sub-pixel range of dimensions may be imperceptible in an output print. However, as is well known in the imaging arts, undesirable banding effects can be the result of a repeated pixel positioning error due to the printhead or its support mechanism. Such banding is typically most noticeable in areas of text or areas of generally uniform color, for example. Manufacturers of inkjet systems recognize that banding effects can severely compromise the image quality of output prints. One solution used to compensate for banding effects is the use of multiple banding passes, repeated over the same area of the printed medium. This enables a printhead to correct for known banding errors, but requires a more complex printing pattern and a more complex medium transport mechanism, and takes considerably more time per print. Under worst-case conditions, correction for band effects can result in significant loss of productivity, even as high as 10× by some estimates.
Even in the case that all nozzles have identical trajectory directions and identical timing responses, there may still be opportunity for improvement of image quality through the control of droplet placement within each pixel, for example to achieve half-toning or to improve the edge resolution of printed text.
It can readily be appreciated that it would be desirable to correct slight dimensional placement errors by controlling the operation of individual nozzles of print head 16, thus obviating the need for multiple banding passes. Proposed solutions for adjusting dot placement with ink jet printing apparatus of various types include the following:    U.S. Pat. No. 6,457,797 (Van Der Meijs et al.) discloses using timing changes to offset the effects of print head temperature changes on relative dot placement for a complete nozzle array in a drop-on-demand type ink jet printer;    U.S. Pat. No. 4,956,648 (Hongo) also discloses manipulating timing intervals for correcting slow and fast scan dot placement in a drop-on-demand type ink jet printer, segmenting the unit dot pitch time interval into suitable sub-intervals;    U.S. Pat. No. 6,536,873 (Lee et al.) discloses bidirectional droplet placement control in a drop-on-demand type ink jet printer, using heater elements in droplet formation;    U.S. Pat. No. 4,347,521 (Teumer) and U.S. Pat. No. 4,540,990 (Crean) discloses a print head employing a complex set of electrodes for droplet deflection in a continuous ink jet apparatus to account for variations in position and drop throw distance.    U.S. Pat. No. 4,533,925 (Tsao et al.) discloses a continuous inkjet printhead assembly in which drops are selectively charged to be deflected perpendicular to nozzle rows by particular amounts. By arranging the nozzle rows skewed with respect to the direction of movement of the medium, drops at any particular location in the printed image may be caused to originate from more than a single nozzle. Artifacts are thereby suppressed by choosing randomly amongst various nozzles.    U.S. Pat. No. 4,384,296 (Torpey) similarly discloses a continuous ink jet print head having a complex arrangement of electrodes about each individual print nozzle for providing multiple print droplets from each individual ink jet nozzle;    U.S. Pat. No. 6,367,909 (Lean) discloses a continuous ink jet printing apparatus employing an arrangement of counter electrodes within a printing drum for correcting drop placement;            U.S. Pat. No. 6,517,197 (Hawkins et al.) discloses an apparatus and method for corrective drop steering in the slow scan direction for a continuous ink jet apparatus using a droplet steering mechanism that employs a split heater element;            U.S. Pat. No. 6,491,362 (Jeanmaire) discloses an apparatus and method for varying print drop size in a continuous ink jet printer to allow a variable amount of droplet deflection in the fast scan direction with multiple droplets per pixel;    U.S. Pat. No. 6,213,595 (Anagnostopoulos et al.) discloses a continuous ink jet apparatus and method that provides ink filament steering at an angle offset from normal using segmented heaters;    U.S. Pat. No. 6,508,543 (Hawkins et al.) discloses a continuous ink jet print head capable of displacing printing droplets at a slight angular displacement relative to the length of the nozzle array, using a positive or negative air pressure;    U.S. Pat. No. 6,572,222 (Hawkins et al.) similarly discloses use of variable air pressure for deflecting groups of droplets to correct placement in the fast scan direction;    U.S. Patent Application No. 2003/0174190 (Jeanmaire) discloses improved measurement and fast scan correction for a continuous ink jet printer using air flow and variable droplet volume;    U.S. Pat. No. 6,575,566 (Jeanmaire et al.) discloses further adaptations for improved print droplet discrimination and placement using variable air flow for each ink jet stream; and    U.S. Pat. No. 4,275,401 (Burnett et al.) discloses deflection of continuous ink jet print droplets in either the fast or slow scan direction using an arrangement of charging electrodes.
As the above listing shows, there have been numerous proposed solutions for correcting print droplet placement in both drop-on-demand and continuous inkjet printing apparatus. Not all of these solutions can be applied to a continuous ink jet printing apparatus, particularly for slight corrections for fast scan placement, for example for corrections in placement less than the center to center spacing of printed drops printed in succession, particularly where such an apparatus does not employ electrostatic forces for droplet deflection. Moreover, taken by themselves, none of these solutions meet all of the perceived requirements for robustness, precision accuracy to within a fraction of pixel dimensions, low cost, compatibility with slow scan adjustment mechanisms, and ease of application and adaptability. In particular, there remains significant room for improvement in implementation of droplet placement in the fast scan (F) direction, that is the direction in which a printhead is typically scanned rapidly across a recording medium. Specifically, there would be particular advantages to a solution that would allow the following:    (a) control of the number of droplets used to form a printed drop printed in a pixel;    (b) precision control of the center (centroid) of each printed drop printed within an associated pixel area, with respect to the fast scan direction; and,    (c) control of the spread of each printed drop printed within an associated pixel area, with respect to the fast scan direction.
In addition, there remains room for improvement in controlling droplet placement in the slow scan direction, and for simple methods that allow control of drop placement in both orthogonal fast and slow scan directions. Prior art solutions which do not rely on complex means of steering drops in the slow scan direction, are unable to correct for placement errors of printed drops in both slow and fast scan directions and thus are unable to place drops at all desired locations within pixels.